Silver cleaner



United States Patent NoDrawinga Application May-8 ,-19 53 i Serial-No. 353,897

7 Claims; (Cl. 251- 152) This invention" relates to cleaning compositions for metals and it has"particu1ar relation to'compositions adapted to remove ,tarn'ish caused by sulfide and oxide films from metal surfaces, patricula'rly surfaces of silver, copper and their'alloys". V The main object of the present invention'is to provide a cleaning composition of the above" mentioned type r which is capable of efliciently removing. sulfide and oxide films from surfaces consisting of silver; copper and their alloys, without corrosion ofor other harmfulinfluence on the metallic surface to be cleaned.

Anotherobject of this invention is to carry out the cleaning operation without the escape of hydrogen sulfide into the atmosphere.

It has been known to use amixture of a colloid, such as evaporated cellulose pulp waste liquor, with diortholylthiourea for selectively controlling the action of an acid pickling bath on metals. Furthermore, it has been suggested previously to add to acid cleaning solutions a substituted thiourea containing a hydrogenated aryl group, as an inhibitor for the acid and it was known to use thioureas for dissolving silver sulfide, for example in the mirror industry. Finally, it has also been suggested to prepare cleaning compositions for silverand copper-containing surfaces, from thiourea and a water-soluble acid forming no water-insoluble compounds with copper and silver.

When thiourea is used in a cleaning and pickling solution, tarnish due to a metal sulfide is effectively removed. However, due to the chemical action of thiourea on the metal sulfide, hydrogen sulfide gas of offensive odor is released during this treatment. This tends to make metal cleaning operations using thiourea unpleasant, and if enough hydrogen sulfide gas is evolved, possibly dangerous.

It has now been found that this essential disadvantage of thiourea can be greatly reduced or eliminated by using a cleaning composition in which thiourea is present as an active ingredient in mixture with a salt of urea and/ or guanylurea wherein HX stands for one mol of the salt-forming acid. The salts of urea and/ or guanylurea used with thiourea according to the present invention have three functions: (a) upon dissolving the solid mixture in Water, they furnish the necessary acidity and form metal pickling and cleaning solutions, which are not corrosive; (b) the salts of urea and/ or guanylurea react with the hydrogen sulfide released in presence of metal sulfides in the cleaning solution and thus almost entirely prevent the escape of H 8 and the appearance of its unpleasant odor; (c) by the reaction of H 8 with the urea and/or guanylurea salts, the latter are converted into the corresponding salts of thiourea and/or guanylthiourea so that said reaction ,results in the formation of additional amounts of thiourea 2,907,716 Patented Oct. 6, 1959 2 compoundswhich supplement and strengthen the action of thiourea originally presentin the solution.

The reaction between H 8 and the salt of urea and guanylurearespectively, is apparently as follows: 1 nm nmnx +12ns nznc aNnmx 1120 Heating of the pickling and cleaning bath, for example from 50 to C, accelerates the above reactions.

Example 1 The following ingredients are mixedto form a uniform powder:

Percent by weight Thiourea 40-60 Urea phosphate 40-60 As an additional ingredient, l-5% by Weight of a conventional wetting agent can be incorporated in the composition.

The composition is preferably used in aqueous solution. For example 5 to 10 parts .by weight of the above comp'ositionare dissolvedin 60 parts by weight of Water and: the solution x is used as a cleaning composition for silver or copper.

i Example. 2

In order to prepare a cleaning solution for silver 10 to 15 parts of the following composition are dissolved in 60 parts of water:

Percent by Weight Thiourea 40-60 Urea oxalate 40-60 If desired, a wetting agent can be incorporated in this composition in the manner and amount stated in Example 1.

Example 3 Percent by weight Thiourea 40-60 Urea phosphate 30-50 Guanyl urea phosphate 5-10 are mixed to a uniform product, which can be used, for example, in aqueous solution containing 8 to 15% by Weight of the solid mixture.

Example 4 Percent by weight Thiourea 40-60 Urea oxalate 30-50 Guanyl urea sulfate 5-10 are homogeneously mixed and dissolved in water to form a solution containing 15 to 20% of the mixture.

It will be understood that this invention is not limited to the specific substances, proportions, procedures and other details specifically described above, and can be carried out with various modifications. Thiourea is the ac tive cleaning ingredient capable of dissolving the metal sulfides and must be present in the composition in an amount sufficient to act on the impurities to be removed. I have found that proportions of 40 to 60% of thiourea based on the weight of the total solid ingredients are satisfactory. On the other hand, the salt or salts of urea and/or guanylurea must be present in a concentration sufficient for reacting with the H S evolved during cleaning. I have found that this concentration should be in the range of 30 to 60% by weight based on the amount of total solid ingredients.

Any inorganic or organic acid salt of urea or guanyl 3 Y urea, which is soluble in water, not oxidizing or toxic, and does not form water-insgluble salts with the metals to be removed, can be used,'e.g. sulfate salts, phosphates, oxalates, 'phthalates, :citrates, succinates, sulfamates and others. A mixture of difierent salts can also be used. Examples of wetting agents which can be used in carrying out the invention are sulfonated fatty-aclohols, sulfonated fatty acid esters, organic acid esters of diethyl{ ene glycol, decyl-be nzene sodium sulfonate, sodium lauryl sulfobenzoate, alkyl naphthalene sodium sulfonate, sulfonated alkyl amides and the like.

The formul as ofthe salts described in the above .examples are:

Urea phosphate:

CO (fish- 13 4 Guanylurea phosphate:

Urea oxalate:

However, watersoluble salts of urea and/ or guanylurea with acids, in which the ratio of urea or guanylurea to the acid is diflYerent from the ratios in the aboveforumlas, can also be used.

The composition according to the invention can be a salt of guanyl urea. Y

, a 4 V used also in paste form, for example in the form of an aqueous paste.

What is claimed is:

1. A cleaning composition for silver, copper and alloys of silver and copper, consisting substantially of thiourea in mixture with-a water-soluble, non-toxic salt, which does not form a water-insoluble salt with silver and copper, of a substance selected from the group consisting ofurea and guanylurea. g

2; The cleaning-composition of claim 1, in which the ratio between thiourea and the ingredients for reacting with H S is in the'range of 30 to by weight.

3. The cleaning composition of claim 1, in which the H S-binding substance is a urea salt, said salt being capable of chemically reacting with-H 8 evolved during cleaning with the composition.

4. The cleaning composition of claim 1, in which the H S-binding substance is-a salt of guanylurea, said salt being capable of chemically reacting with H S evolved during cleaning with the composition.

'5. The cleaning composition of claim 1, in which the H s-binding substance is a mixture of a salt of urea with 6. The cleaning composition of claim 1, in the form of a solid mixture. I

7. The cleaning composition, of claim 1, in the form of an aqueous. solution.'

References Cited in the file of this patent UNITED STATES PATENTS 

1. A CLEANING COMPOSITION FOR SILVER, COPPER AND ALLOYS OF SILVER AND COPPER, CONSISTING SUBSTANTIALLY OF THIOSREA IN MIXTURE WITH A WATAER-SOLUBLE, NON-TOXIC SALT, WHICH DOES NOT FORM A WATER-INSOLUBLE SALT WITH SILVER AND COPPER, OF A SUBSTANCE SELECTEDFROM THE GROUP CONSISTING OF UREA AND GUANYLUREA. 